1. Field of the invention
The present invention, generally, relates to the packaging of electronic devices and, more particularly, to a first level electronic package, and method of making the same, for carrying an electronic device.
In recent years, the electronics industry has grown, improved and developed in its advance of the technology that it uses, and the technology involving the packaging of circuit components is advancing now with matching speed. For example, one monolithic chip measuring less than one-half inch square today can store one million bits of information, and in addition, such a chip can have many electronic circuits. Placing more and more electronic circuits on smaller and smaller chips makes the task of packaging these smaller chips with a module, approach the impossible.
A major restriction in attaching a device to a module seems to arise because of the mismatch in the respective coefficients of thermal expansion. This mismatch causes fractures and other circuit discontinuities in the electrical interconnections during thermal cycling. For example, the coefficient of thermal expansion for a typical chip is in the order of 3 microinches per inch per degree Centigrade, while that of a typical ceramic module is in the order of 7 microinches per inch per degree Centigrade.
These fractures and other circuit discontinuities are the results of stresses that are developed when there is thermal cycling involved in their environment. This problem is compounded even further when a device is affixed rigidly to a module, as they are so frequently. The prior art is rich with efforts to solve the problems that arise from these thermal mismatches.
2. Description of the prior art
U.S. Pat. No. 3,777,220 to Tatusko et al. teaches a unique structure formed of discrete areas with different coefficients of thermal expansion and suggests the attaching of devices to those areas more closely matching the coefficient of thermal expansion of the device. This may well be a great teaching, theoretically, but with today's subminiturized devices, it lacks practicality.
U.S. Pat. No. 3,969,754 to Kuniya et al. describes an arrangement whereby a supporting electrode has embedded fibers of a coefficient of thermal expansion "substantially equal to" that of a device to be attached. The idea may be sound, but it lacks a degree of universal applicability that is necessary to attract financial investors in today's market.
U.S. Pat. No. 4,189,524 to Lazzari describes a multilayer, rigid support composed of a very thin layer of conductive material with negligible thermal expansion inserted between each conductive layer and each insulating layer. When this prior patent teaches that the problem of elevations of temperatures "can be ignored", it leads diametrically opposite to the present invention.
U.S. Pat. No. 4,413,308 to Brown clearly recognizes the problem of providing a support for a chip to prevent solder joint failures due to stress and differential thermal expansion, but the solution that is suggested is only limited and incomplete as it concerns the continuing problem in the manufacture of electronic equipment using microcircuit chips.
The problem arises because of the requirement of mounting, or packaging, the chip in such a way as to provide adequate heat sinking of the chip while permitting ease of mechanical and electrical connection, and disconnection, between the package and its printed circuit board. Underlying the problem is the thermal incompatibility of the carrier material, usually ceramic in nature, and the typical printed circuit board, usually an epoxy or the like material.
A direct mounting of a chip carrier to a printed circuit board produces intolerable differential expansion that causes the above-described fractures or other discontinuities as a result of the stresses that are developed in these mounting connections during the normal cycles in temperature. As the above referred-to prior art patents indicate, there have been numerous approaches attempted in the past to solve this on-going problem, but until the present invention, none has appeared as totally effective.